1. Field of the Invention
The present invention generally relates to a speaker apparatus. The present invention specifically relates to a speaker apparatus having a flux applicator that applies magnetic flux to a voice coil arranged in a magnetic gap.
2. Description of the Related Art
In a so-called external magnet speaker apparatus, a center pole is disposed at the center of the apparatus, a ring magnet that is a permanent magnet is coaxially disposed around the center pole, and a top plate that forms a magnetic circuit along with the ring magnet is disposed on the ring magnet. Thus, a magnetic gap is formed between the top plate and the center pole. A voice coil is arranged in the magnetic gap such that the voice coil can vibrate. The voice coil is connected to a diaphragm. When audio signal current is supplied to the voice coil while the voice coil is applied with magnetic flux via the top plate, the diaphragm vibrates whereby voice and sound are reproduced.
The top plate and the center pole configure a flux applicator that applies the magnetic flux to the voice coil. The top plate disposed radially outside the voice coil configures an outer flux applicator, and the center pole disposed inside the voice coil configures an inner flux applicator.
It is necessary that the length of the magnetic gap along the direction of vibration of the voice coil be longer than a stroke of the voice coil. Therefore, the top plate requires an opposing surface with a prespecified length in the direction of vibration of the voice coil i.e., axial direction of the center pole. Namely, the top plate requires a prespecified thickness in the axial direction. Especially, a bass speaker apparatus, which has a longer stroke, requires a magnetic circuit having a longer opposing surface. Namely, a so-called long-gap magnetic circuit is required.
On the contrary, there has been proposed a technique of forming an opposing surface by just using a thicker top plate. However, a thicker top plate makes the speaker apparatus heavier, bigger, and costlier.
One approach is to punch a ring part from a soft magnetic material thin plate and bend the inner edge of the ring part to be double to obtain a top plate having the required thickness. A conventional technology has been disclosed in, for example, Japanese Patent Publication No. HEI5-168092. This approach may solve the issues of higher weight and bigger size of the apparatus; however, it does not solve the issue of higher costs because it requires technically difficult machining processes to be carried out.
Moreover, in the conventional speaker apparatus, a large amount of heat is generated due to vibrations of the voice coil. The heat is conducted to the opposing top plate and heats up the top plate. A hot top plate can cause malfunctions thereby reducing the durability of the apparatus. Therefore, there is a need to appropriately release the heat. The issue of the heat becomes severe in a bass speaker apparatuses, in which high currents are applied to the voice coil, and in in-vehicle speaker apparatuses, which are placed in a small space.